1. Field of the Invention
The present invention relates to an apparatus and method for controlling a gray level for a display panel.
2. Description of the Related Art
With recent tendency of large sized display devices, request of flat display panels occupied by a small space is increasing. As one example of the flat display panels, an organic EL display panel is receiving much attention. The organic EL display panel is very thin and can be fabricated in a matrix arrangement. Also, the organic EL display panel can be driven at a low voltage of 15V or less.
Recently, an active driving method is mainly used in the organic EL display panel. That is, in the active driving method, charges are stored in a capacitor using a low current and a driving transistor is driven using the charges of the capacitor.
In the active driving method, a gray level for a display panel is controlled by controlling the range of a driving current. However, since the range of the driving current is very small within several hundred pA to several tens of nA, it is difficult to control the gray level for the display panel by appropriately controlling the driving current.
Also, since the very small driving current is used, a charge storage capacitor requires a value of very small capacity. However, there is a limitation in reducing the size of the capacity of the charge storage capacitor due to leakage generated by a switching transistor of the organic EL display panel.
In other words, if leakage current generated by the switching transistor is great, the capacitor requires a greater capacity than the leakage amount to set a desired gray level in the display panel.
However, problems arise in that the pixel size of the display panel is limited and the size of the capacitor is also limited by the limited pixel size. To solve such problems, it is necessary to increase the pixel size or minimize the leakage current. However, it is general tendency that efforts for forming a small sized pixel are in progress, and there is still a limitation in reducing the leakage current.